The present invention relates to an anti-horizontal-impulse cathode-ray-tube driving system, and more particularly to such a driving system having an additional coil added to the horizontal output transformer of the cathode-ray-tube and controlled to produce an induced voltage, in phase reverse to the upper horizontal impulse of the anode high voltage, connected between the high voltage generator and the anode so as to eliminate the horizontal impulse from the display of the magnetic deflection type of cathode-ray-tube and the radiation of the alternating electrical field derived therefrom.
The known magnetic deflection type cathode-ray-tube driving systems are generally gathered into two types, namely, the compound type and the separated type, as shown in FIGS. 1 and 2, according to whether the high voltage is derived from the horizontal output or from a separate generator. In either type, a horizontal deflection yoke (HDY) (21) is mounted around the neck portion of the cathode-ray-tube (2), and driven by the horizontal output (1) to provide the cathode-ray-tube (2) with a horizontal deflection field. In a compound type cathode-ray-tube driving system, anode high voltage is simultaneously provided by the horizontal output (1). While in a separated type cathode-ray-tube driving system, anode high voltage is provide by a high voltage generator (3). Using a high voltage generator to provide an anode high voltage increases the cost of a cathode-ray-tube driving system, but ensures better quality. FIG. 1 illustrates a compound type cathode-ray-tube driving system according to the prior art, in which the circuit which provides an anode high voltage is incorporated into the circuit of the horizontal output. This design reduces the manufacturing cost of a driving system, however, its wide anode high voltage load regulation affects the stability of the raster produced in conjunction with the horizontal deflection yoke. FIG. 2 illustrates a separated type cathode-ray-tube driving system according to the prior art, in which an independent high voltage generator (3) is provided to produce an anode high voltage. This design ensures a cathode-ray-tube to produce a more stable raster. However, the arrangement of the high voltage generator (3) makes the structure complicated, and simultaneously increases the manufacturing cost of the driving system.
In either of the aforesaid two prior art cathode-ray-tube driving system, an alternating electrical field (AEF) is produced between the anode (22) of the cathode-ray-tube (2) and the earthed external graphite layer (23). As illustrated in FIG. 3, a capacitor Cy is formed between the horizontal deflection yoke (21) and the anode (22), and another capacitor Ct is formed between the graphite layer (23) and the anode (22). The two capacitors Cy,Ct form into a series circuit to bear the voltage Vce at the horizontal deflection yoke (21), as shown in FIG. 4. Therefore, ##EQU1## in which, the voltage Vcy at the capacitor Cy is limited to the area between the horizontal deflection yoke (21) and the anode (22), and hardly affect the physiological functions of the people or the operation of the electric appliances nearby. On the contrary, the voltage Vct at the capacitor Ct is superimposed on the anode high voltage (4), and therefore an impulse (41) is formed, as shown in the dotted lines in FIG. 5. When connected to earth, the impulse forms into an alternating electrical field (AEF) around the cathode-ray-tube. Because the alternating electrical field is radiative, it is harmful to the physiological functions of the people nearby, and interferes with the operation of other electric appliances. Because the phase and cycle of the impulse are approximately equal to the voltage Vce at the horizontal deflection yoke (21), the impulse is called synchronous horizontal impulse or horizontal impulse.
In order to eliminate the aforesaid alternating electrical field, a Farady's shield may be used. Originally, a Farady's shield was made from a metal material and used to isolate the radiation source from the outside. In recent years, plastic materials are used in making a Farady's shield with the inside surface thereof coated with a conductive material. After connected to earth, the outside surface of a Farady's shield forms an earth potential. This method is just to block up a radiation source but not to completely eliminate it. Furthermore, the installation of a Farady's shield complicates the manufacturing process of a cathode-ray-tube driving system, and simultaneously increases its manufacturing cost.